Two-stage lime mud calciner

ABSTRACT

A two-stage system for calcining lime mud. In the first stage the lime mud is mixed with kraft black liquor and dried in a fluidized bed or spray dryer, using as a source of heat for drying, the hot exit gases from the second stage. In the second stage, the dried mixture of mud and black liquor solids is calcined in a fluidized bed operation carried on at temperatures between 1400* and 1900* F. The dried black liquor solids are used as fuel for the calcining operation. The hot exhaust gases from the second stage calcining operation are recycled to the first stage and are used to dry the kraft black liquor in the first stage drying system. Calcined product is continuously removed from the second stage calcining operation and conveyed back to the recausticizing system for reuse in the process.

United States Patent 451 June 13, 1972 Copeland TWO-STAGE LIME MUDCALCINER [72] Inventor: George G. Copeland, Western Springs, Ill. [73]Assignee: Copeland System Inc., Oak Brook, Ill.

[22] Filed: Aug. 13, 1970 [2]] Appl. No.: 63,544

[52] 11.8. CI. ..263/21 A, 263/53 R [51] Int.Cl ..F27b 15/00 [58] FieldofSearch ..263/53 R, 21 A [5 6] References Cited UNITED STATES PATENTS2,650,084 8/1953 White ..263/53 3,515,381 6/1970 Foch ..263/21 A3,127,455 3/1964 Culbertson, Jr. et a1. ..263/53 2,789,034 4/1957 Swaineet al ..263/21 A Primary Examiner-John J. Camby Attorney-Merriam,Marshall, Shapiro & Klose [57] ABSTRACT A two-stage system for calcininglime mud. In the first stage the lime mud is mixed with kraft blackliquor and dried in a fluidized bed or spray dryer, using as a source ofheat for drying, the hot exit gases from the second stage. In the secondstage, the dried mixture of mud and black liquor solids is calcined in afluidized bed operation carried on at temperatures between l400 and 1900F. The dried black liquor solids are used as fuel for the calciningoperation. The hot exhaust gases from the second stage calciningoperation are recycled to the first stage and are used to dry the kraftblack liquor in the first stage 'drying system. Calcined product iscontinuously removed from the second stage calcining operation andconveyed back to the recausticizing system for reuse in the process.

6 Claim, 1 Drawing Figure souo PRODUCT PATENTEDJUN 1 s 1912 3. 669,430

:1 LIME MUD BLACK LIQUOR /9 GAS 27 1 23 t 7' souo PRODUCT AIR I J J j#24 INVENTOR ATTORNEYS TWO-STAGE LIME MUD CALCINER This inventionrelates to an improved lime mud calcining system which is particularlyadaptable to recausticizing systems in kraft pulp mills. Moreparticularly the invention relates to a two-stage system for calcininglime mud which employs no extraneous fuel and which therefore permitssubstantial economies to be effected in the operation of the system.

In the conventional operation of a sulfate (kraft) liquor recoverysystem, green liquor is obtained by dissolving in water the smeltproduced in the black liquor recovery system. The green liquor, whichcontains substantial quantities of dissolved sodium carbonate, isclarified to remove any insoluble residue, then treated with lime(causticized) to produce a precipitate (lime mud) consisting primarilyof calcium carbonate, while regenerating sodium hydroxide. Theprecipitated lime mud is separated from the solution (white liquor)containing the regenerated hydroxide, which is recycled for reuse.

The lime mud, after washing to remove entrained white liquor, is driedand calcined for regeneration of lime to be reused in the process. Thecalcining operation is typically carried out in a rotary cylindricalsteel kiln which is lined with refractory material. In order to causethe dissociation of calcium carbonate (CaCO into lime (CaO), thetemperature in the calciner must reach at least about 1600 F. It isconventional to supply a stream of hot gas at temperatures up to about2400 F. to one end of the calciner, while the lime mud enters the otherend, and moves countercurrently to the hot gas stream. Effective heatexchange between the countercurrent streams of gases and solids withinthe calciner is relatively difficult to achieve in this operation.Accordingly, the exit gases and reconstituted lime typically leave thecalcining operation at elevated temperatures and contain substantialquantities of sensible heat which is either lost or incompletelyrecovered. In addition, the energy necessary to heat the entering streamof hot gas to the high temperature required for effective calcination isan important factor in the overall cost of the operation.

In accordance with the present invention, there is provided a two-stagesystem for calcining lime mud. In the first stage the lime mud is mixedwith kraft black liquor and dried in a fluidized bed or spray dryer,using as a source of heat for drying, the hot exit gases from the secondstage. In the second stage, the dried mixture of mud and black liquorsolids is calcined in a fluidized bed operation carried on attemperatures between 1400 F and l900 F. The dried black liquor solidsare used as fuel for the calcining operation. The hot exhaust gases fromthe second stage calcining operation are recycled to the first stage andare used to dry the kraft black liquor in the first stage drying system.Calcined product is continuously removed from the second stage calciningoperation and conveyed back to the recausticizing system for reuse inthe process.

The use of combustible black liquor solids as fuel for the calciningoperation eliminates the need for any extraneous fuel, thus reducing thecost of the operation. In addition, a possible overloaded condition ofthe black liquor recovery is relieved by the diversion of some of theblack liquor to the calcining operation.

The invention will be better understood from the following detaileddescription thereof taken in conjunction with the accompanying drawing,which is a diagrammatic representation of a typical embodiment of theinvention.

The following description will be that of the operation of a typicalsystem of the invention in which the stated quantities of reactants arethose required to produce one ton of regenerated lime. Referring to thedrawing, the system consists of an enclosed tower 10 divided byseparator 11 into upper and lower zones, 12 and 13 respectively. Duringsteady state operation of the system upper zone 12 contains a fluidizedbed 14 of particles of lime mud which are introduced (6037 lb., 65percent solids, 75 F.) into the system through conduit 16. The bed 14 ismaintained in a fluidized condition by means of a flow of gas (9081 lb.)rising upwardly from the bottom of zone 12 through distributor plate 17and thence upwardly through the bed 14. The gas which is supplied to thebottom of zone 12 is at an elevated temperature l200 F to l800 F and inaddition to maintaining fluidizing conditions in zone 12 dries theparticles of lime mud of any moisture contained therein.

Also introduced to zone 12 through line 18 is black liquor (2,012 lbs.,60 percent solids of which 55 percent are inorganic, F.) which issprayed through nozzle 19 onto the particles in bed 14. The solidspresent in the black liquor have a heat value of about 6600 BTU/lb. Thedrying action of the gas in the fluidized bed deposits the solidmaterials in the black liquor onto the particles of lime mud and therapid mixing action of the fluidized bed insures a relativelyhomogeneous dispersion of the black liquor solids.

A stream of dry lime mud particles (5131 lb., 250 F.) containing theblack liquor solids deposited thereon is conveyed via conduit 21 to thelower zone 13 of vessel 10. Not shown in the drawing is conventionalsolids conveying apparatus which may be necessary for this transfer andwhich will be apparent to those skilled in the art. In this connectionthe vertical positioning of zone 12 above zone 13 in a single vessel isadvantageous in permitting gravity to aid the transfer of the solidparticles from zone 12 to zone 13. It will be apparent that the processcould alternatively be carried out with these two zones in separatevessels at the same elevation. In such case, however, additional solidsconveying apparatus will probably be required. I

The particles of dried lime mud and black liquor solids entering zone 13are maintained in a fluidized bed 22 by means of an upwardly movingstream of air which enters the bottom of the vessel through conduit 23and passes through the distributor plate 24. As is the case in zone 12,the stream of air has a velocity which is correlated with the size ofthe particles in bed 22 to maintain a fluidized condition therein. Forthe production of one ton of regenerated product, about 7085 pounds ofair is introduced through conduit 23, at a temperature of 200 F. Thecombustion of the black liquor solids within zone 22 in conjunction withthis amount of air creates a temperature of about 1400 F to l900 Fwithin the zone, which causes the regeneration of calcium oxide (lime)from the calcium carbonate which is fed thereto.

The gases leaving bed 22 (9,081 lb., l600 F.) are conveyed by conduit 24to the bottom of zone 12 where they are used to fluidize and dry theentering lime mud particles and black liquor in the manner previouslydescribed. The exit gases leaving zone 12 via conduit 26 (10,999 lb.,250 F.) are conveyed to a scrubbing system, not shown, and thence to theatmosphere.

The desired regenerated lime product is recovered from zone 13 viaconduit 27 in a total quantity of about 3,017 lb., of which one ton islime, the remainder being inorganic and inert materials introduced inthe black liquor.

The system described above can be varied without departing from thescope ofthe invention. As previously explained, the elevated position ofthe drying zone relative to the calciner is desirable but not necessary.Similarly, although a fluid bed dryer is desirable, another type ofdirect contact heat exchanger, such as a spray type dryer, could also beused.

The foregoing detailed description has been given for clearness ofunderstanding only, and no unnecessary limitations should be understoodtherefrom, as modifications will be obvious to those skilled in the art.

lclaim:

l. A process for calcining lime mud from kraft pulping operations toregenerate lime therefrom which comprises mixing said lime mud withblack liquor containing combustible solids, drying said mixture bydirect heat exchange with a' stream of hot gases recycled from asubsequent step in the process to produce dried lime mud particlescarrying combustible black liquor solids, creating a fluidized bed ofsaid lime mud particles at elevated temperatures by means of afluidizing stream of airwhich also causes combustion of said blackliquor solids within said fluidized bed, the quantity of fluidizing airbeing correlated with the quantity of black liquor solids to producecalcining temperatures in said bed sufficient to regenerate lime fromsaid lime mud, and withdrawing from said calcining step a solid productcontaining regenerated lime and a stream of hot gases which is recycledto said drying step.

2. The process of claim 1 wherein said mixture of lime mud and blackliquor is dried in a bed maintained in a fluidized state by saidrecycled stream of hot gases.

3. The process of claim 1 wherein said calcining temperature is at leastabout 1400 F 4. The process of claim 1 wherein said drying step iscarried at a physical elevation higher than said calcining step, wherebythe transportation of said dried particles from said drying step to saidcalcining step is effected at least partially by the influence ofgravity.

5. Apparatus for calcining lime mud from kraft pulping operationscomprising a vessel having an upper drying zone and a lower calciningzone,

said upper drying zone comprising means for spraying black liquorcontaining combustible solids onto a bed of lime mud particles, 7

means for drying said lime mud particles by direct contact with a heatedgas produced in said lower calcining zone to produce dry particles oflime mud carrying combustible black liquor solids,

means for transporting said dry particles to said lower calcining zone,said lower calcining zone comprising fluidbed calcining means forcreating and maintaining a fluidized bed of said lime mud particles,

means for introducing a stream of fluidizing air to said fluidized bed,said air also serving as combustion air for calcining said particles,

means for collecting heated gas produced in said calcining zone andconveying said gas to said upper drying zone, and

means for collecting and recovering regenerated lime from said calciningzone.

6. Apparatus in accordance with claim 5 wherein said upper drying zonecomprises a fluid-bed drier.

* 1B i l

2. The process of claim 1 wherein said mixture of lime mud and blackliquor is dried in a bed maintained in a fluidized state by saidrecycled stream of hot gases.
 3. The process of claim 1 wherein saidcalcining temperature is at least about 1400* F
 4. The process of claim1 wherein said drying step is carried at a physical elevation higherthan said calcining step, whereby the transportation of said driedparticles from said drying step to said calcining step is effected atleast partially by the influence of gravity.
 5. Apparatus for calcininglime mud from kraft pulping operations comprising a vessel having anupper drying zone and a lower calcining zone, said upper drying zonecomprising means for spraying black liquor containing combustible solidsonto a bed of lime mud particles, means for drying said lime mudparticles by direct contact with a heated gas produced in said lowercalcining zone to produce dry particles of lime mud carrying combustibleblack liquor solids, means for transporting said dry particles to saidlower calcining zone, said lower calcining zone comprising fluid-bedcalcining means for creating and maintaining a fluidized bed of saidlime mud particles, means for introducing a stream of fluidizing air tosaid fluidized bed, said air also serving as combustion air forcalcining said particles, means for collecting heated gas produced insaid calcining zone and conveying said gas to said upper drying zone,and means for collecting and recovering regenerated lime from saidcalcining zone.
 6. Apparatus in accordance with claim 5 wherein saiduPper drying zone comprises a fluid-bed drier.